The Australian currency was lower at midday, as investors worry the troublesome resource super profits tax (RSPT) will return on the agenda if the Greens hold the balance of power after the August 21 federal election.

At 1200 AEST Thursday, the domestic unit was exchanging at $US0.8969/73, a decline from yesterday's finish of $US0.9005/08.

Since 0700 AEST Wednesday, the Aussie ranged from $US0.8951 to $US0.8986.

The local dollar weakened against most major currencies, despite investors avoiding the greenback during the offshore trade, according to Tallship Investments portfolio manager Michael Katz.

"There's probably a bit of worry about the Greens holding the balance of power and fears the mining tax could be back on the agenda,'' he said.

Greens leader Bob Brown wants to revive Labor's original 40 per cent RSPT proposal to generate extra revenue to be used in funding $2 billion in public-school programs.

"The Greens support the mining super profits tax as originally proposed by Wayne Swan and Treasury,'' the senator said yesterday.

While the odds may favour the return of a Gillard government, the Greens are polling as much as 14 per cent in the surveys. This suggests the minor party will win more seats in the upper house in addition to their current five members, and obtaining sufficient seats in the Senate will give significant negotiating clout on a range of issues.

Their stand on the levy would, however, likely reignite the miners' campaign against the resource tax, which the Coalition pledges to discard completely.